Mask frame



FIG. 1 is a front perspective view of a mask frame according to anembodiment of our design;

FIG. 2 is a front view thereof;

FIG. 3 is a rear view thereof;

FIG. 4 is a top view thereof;

FIG. 5 is a bottom view thereof; and,

FIG. 6 is a is a right side view thereof with the left side view being amirror image thereof.

Where included, broken lines showing environment or structure are forillustrative purposes only and form no part of the claimed design.

The ornamental design for a mask frame, as shown and described.